Combined cycle power plants are known in the art as an efficient means for converting fossil fuels to thermal, mechanical, and/or electrical energy. Combined cycle power plants are known to include a gas portion and a steam portion. Thermodynamically, the gas portion operates as a Brayton cycle and the steam portion operates as a Rankine cycle. The gas portion, or topping cycle, includes a gas turbine engine powered by the combustion of a fuel such as natural gas or fuel oil. A steam turbine of the steam portion, or bottoming cycle, is powered by steam that is generated by the cooling of the gas turbine exhaust in a heat recovery steam generator (HRSG). The HRSG is a heat exchange device that uses the hot exhaust from the topping cycle to generate steam for use in the bottoming cycle. In the combined cycle power plant, the bottoming cycle recovers waste heat from the topping cycle to generate electricity and/or mechanical energy. The steam portion includes an air cooled condenser for converting expanded steam exhausted from the steam turbine into condensate, which is returned to the HRSG for reheating and use in the cycle once again. When converting the expanded steam to condensate, combined cycle power plants can reject a large percentage of heat input as waste heat with nearly 80% of the heat rejection occurring in the condenser of the steam portion. For example, expanded steam can be exhausted from a super-atmospheric steam turbine to the condenser at 16 psia and 220° F. and returned to the HRSG at a sub-atmospheric pressure and reduced temperature. The pressure drop and reduction in temperature are recognized as a source of energy presently unused in combined cycle power plants.
Various technologies have been employed to augment the power output that the gas turbine is able to produce. One technique is to cool the gas turbine inlet air prior to compressing it in the compressor, which increases the turbine's efficiency. Cooling causes the inlet air to have a higher density, thereby creating a higher mass flow rate through the gas turbine. The higher the mass flow rate through the gas turbine, the more power the gas turbine can produce. Evaporative cooling is one method of cooling the gas turbine inlet air.
Evaporative cooling is a technique that has been used for cooling and has been designed for and installed with new power generators, most commonly in the Southwestern United States. Ambient air drawn in from outside transfers heat (i.e., heat of vaporization) to cooler water circulating through a media and, as a result, the temperature of the air is lowered in a process referred to as adiabatic saturation. Because of the still widely held view that evaporative cooling is of little benefit in geographic regions subject to damp weather or high humidity conditions, its use to date has been mainly restricted to providing supplemental cooling for generators located in arid regions, such as the Southwestern United States. The term “arid” used throughout this application is understood to refer to environments having an ambient relative humidity less than about 60% and an ambient temperature greater than about 85° F.
Water or steam injection is yet another method that can be used for power augmentation of the gas turbine. Water injection or steam injection within the combustion chamber, or water added at the compressor inlet when the gas turbine is operating under full load, will increase the power output of a gas turbine above the normal output. As with evaporative cooling, water or steam injection creates a higher mass flow through the gas turbine. This is because mass is added to the flow as steam or water, and the increased power output is the direct result of the increased mass flow. However, the water consumed both by evaporative cooling and steam injection techniques is not recovered and typically is rejected directly to the environment. While this may be acceptable where water is in ample supply, it is an increasing problem in arid or other regions where water is in short supply and is accordingly a valuable commodity. Waste of water in such regions is environmentally irresponsible. Further, the cost of water used in evaporative cooling and steam injection techniques for augmenting electrical output can be significant, and it is becoming increasingly difficult in some areas to obtain a permit to use water for power generation, especially in dry and arid regions.
Accordingly, a need exists for a combined cycle power plant that efficiently augments power output through inlet air cooling without an increase in consumption of plant water. Also, there is a need for a combined cycle power plant that advantageously recovers and uses rejected heat in the bottoming cycle of the combined cycle system.